sysroot: Silence clang-analyzer warning

I think this is another case of clang-analyzer not understanding
the "GError rules"; it's getting confused by the `NULL` checks here
and thinking that if `authority == NULL` it's possible `error == NULL`
too.  But it can't see into the implementation of libpolkit.

src/daemon/rpmostreed-os.cxx

@@ -108,6 +108,7 @@ os_authorize_method (GDBusInterfaceSkeleton *interface, GDBusMethodInvocation *i
       = rpmostreed_sysroot_get_polkit_authority (sysroot, &local_error);
   if (!authority)
     {
+      g_assert (local_error);
       g_dbus_method_invocation_return_error (invocation, G_DBUS_ERROR, G_DBUS_ERROR_FAILED,
                                              "Failed to load polkit: %s", local_error->message);
       return FALSE;

src/daemon/rpmostreed-sysroot.cxx

@@ -529,6 +529,7 @@ sysroot_authorize_method (GDBusInterfaceSkeleton *interface, GDBusMethodInvocati
           = rpmostreed_sysroot_get_polkit_authority (self, &local_error);
       if (!authority)
         {
+          g_assert (local_error);
           g_dbus_method_invocation_return_error (invocation, G_DBUS_ERROR, G_DBUS_ERROR_FAILED,
                                                  "Failed to load polkit: %s", local_error->message);
           return FALSE;